Use was made of inorganic semiconductors, such as gallium arsenide, in the development of light-emitting diodes for electronics or photonics. In addition to semiconductor light-emitting diodes, organic LED's (OLED's) based on vapor-deposited or solution-processed organic compounds of low molecular weight were developed. Recently, conjugated oligomers and polymers based on e.g. substituted p-divinylbenzene, poly(p-phenylenes) and poly(p-phenylenevinylenes) (PPV), polyfluorenes, and poly(spirofluorene)s were described for the manufacture of a polymer LED (polyLED). By ‘conjugated polymer’ is meant a polymer which possesses a delocalized π-electron system along the polymer backbone; said delocalized π-electron system conferring semiconducting properties on the polymer and giving it the ability to transport positive and negative charge carriers with high mobilities along the polymer chain. Such polymers are discussed, for example, by R. H. Friend in Journal of Molecular Electronics 4 (1988) January-March, No. 1, pages 37 to 46.
Until now the most primary organic LED device have comprised a single organic light-emitting layer which is interposed between a transparent electrode as the anode and a metal electrode as the cathode. The organic LED device may have two additional organic layers in order to enhance its emission efficiency, the first layer as a hole transport layer and the second layer as an organic light-emitting layer, or the first layer as an organic light-emitting layer and the second layer as an electron transport layer. These two organic layers are interposed between a transparent anode and a metal cathode. Furthermore, there are devices having three organic layers: the hole transport layer, the organic light-emitting layer, and the electron transport layer in that order, which are interposed between the two electrodes.
In a typical device, a polyLED comprises a hole transport layer, for example a conjugated polymer layer, and a layer of a light-emitting polymer (LEP). The charge mobility of the LEP generally is a compromise between low power, favoring high mobility, and high efficiency, favoring low mobility. The charge mobility of the charge transport layer typically is at least one order of magnitude greater than the charge mobility of the light-emitting layer.
Spin casting is preferably used for the application of the different layers. A major problem for polymer-based multilayer devices is the solubility of the materials used; a multilayer cannot be realised if a spin-cast first layer dissolves in the solvent of a subsequent second layer. As a first approach, efficient bi-layer devices have been realized by N. C. Greenham et al., Nature 1993, 365, 628, using a precursor PPV as hole transport layer, which is insoluble after conversion. Another approach to overcome the solubility problem is to crosslink the first (hole transport) layer after deposition. However, the long UV exposure and reactive end groups needed for crosslinking strongly decreases the performance of LEDs fabricated from these materials as described by B. Domercq et al. in J. Polym. Sc., Part B: Polym. Phys. 2003, 41, 2726. Therefore, there is a need for materials for light-emitting and charge transport layers such that a spin-coated first layer does not dissolve in the subsequently deposited second layer.